The effects of trace elements on grain boundaries were investigated by analyzing the feature and com-position of the boundary precipitates. The effect of trace elements segregated towards boundaries and the mechanism ofinteraction were interpreted by an idea of atom misfit. The mechanisms with regard to spheroidizing and degenerating ofthe precipitates were expounded. For the superalloys studied, the atoms with larger misfit may hinder the atoms withsmaller misfit from segregating towards the boundaries and may drive the latter into the lattice(or interstice) inside grainsand the boundary precipitates. The reaction of the latter on the former was also studied.